1. Field of the Invention
The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for allowing pods to be buffered within a minienvironment adjacent a process tool.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 0.5 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called xe2x80x9cbottom openingxe2x80x9d pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide xe2x80x9cfront openingxe2x80x9d pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell. For both front opening and bottom opening pods, a pod door includes an interior surface which is included as part of the sealed pod environment, and an exterior surface which is exposed to the environment of the wafer fab.
In conventional SMIF systems, in order to transfer workpieces such as semiconductor wafers between a SMIF pod and a process tool within a wafer fab, a pod is typically loaded either manually or automatedly onto a load port of a minienvironment on the front of the tool. Thereafter, mechanisms within the load port decouple the pod shell from the pod door, and then mechanisms within the SMIF interface separate the shell from the door to allow transfer of the cassette and/or wafers. A workpiece handling robot thereafter transfers the workpiece(s) to and from the process tool for processing. After processing of the workpiece(s) at the tool is finished, and the workpiece(s) have been returned to the pod, the SMIF interface thereafter couples the shell and door together, and the pod is transferred from the load port so that the next pod may be brought to the tool and the process repeated.
Presently, a semiconductor wafer fab may cost in excess of $1 billion to outfit, and approximately 80% of that cost is the cost of process tools. It is therefore desirable to maximize the utilization of these tools, and substantial efforts are devoted to minimizing the time that the tools sit idle. In order to prevent significant idle time, it is known to include a local tool buffer adjacent the tool load ports at one or more of the process tools. A local tool buffer allows pods to be stored locally adjacent the tools and quickly transferred to the tool load port without having to constantly retrieve a pod from a remotely located stocker, or depend on timely delivery therefrom. A conventional local tool buffer is shown generally at 10 adjacent a process tool 12 in FIG. 1. As shown therein, a pod handling robot 14 is capable of transferring pods 16 between a plurality of local shelves 18 and the tool load ports 20 on the process tool 12.
Conventional local buffers, such as that shown in FIG. 1, have several shortcomings. First, they take up a significant amount of space within a wafer fab, which space is at a premium. Second, even though local tool buffers are able to supply pods to a load port in a timely manner, valuable time is still spent separating the cassette from within the pod upon initial loading of the pod on the load port, as well as when returning the cassette to the pod after processing of the workpieces in that cassette has been completed. The processing tool may be sitting idle during this time. It is known to provide two load ports on a process tool, so that a cassette may be separated from or returned to a pod on the first port while processing on workpieces from a pod on the second port is taking place. However, it is not feasible to provide a second load port on certain process tool configurations. Additionally, processing tools which are able to support two load ports require duplicate componentry for each load port, thus raising the cost and complexity of operation. Further still, two load ports take up additional space on the front end of the process tool, which space is at a premium.
It is therefore an advantage of the present invention to provide a cassette buffer for storing two or more cassettes within a minienvironment of the SMIF interface affixed to the process tool.
It is a further advantage of the present invention to allow loading of a new workpiece cassette into a process tool as soon as processing on the previous workpiece cassette has been completed.
It is a still further advantage of the present invention to decouple the process of cassette loading into the process tool from the delivery sequence of pods to the process tool.
It is another advantage of the present invention to utilize mechanisms that are presently in use for cassette loading in performing the additional function of improving tool throughput.
It is a further advantage of the present invention to operate with so-called xe2x80x9cSMART tagxe2x80x9d technology so that workpiece lots may be brought to a load port in one pod and transferred away in a second pod, without losing any identification information relating to that particular workpiece lot.
These and other advantages are provided by the present invention which in preferred embodiments relates to a system for buffering two or more cassettes within a minienvironment affixed to a process tool. The present invention is provided as part of a SMIF interface mounted on a frame affixed to the front end of a process tool. The interface includes a load port formed of a port door and a port plate circumjacent thereabout, and a minienvironment mounted to the port plate. The load port is provided for receiving a cassette-carrying pod and decoupling the pod shell from the pod door. Thereafter, translation mechanisms within the interface raise the port plate, the pod shell which is supported on the port plate, and the minienvironment. The port door with the workpiece cassette supported thereon remains stationary as the port plate moves upward.
According to the present invention, as processing occurs on wafers of a first cassette, at least a second cassette may be separated from its pod and stored either in or adjacent the process tool. Thus, upon completion of processing on the wafers of the first cassette, the cassettes may be interchanged quickly to minimize the idle time of the tool. In a preferred embodiment, the shelf may be mounted within the SMIF interface frame, at a height that allows a cassette to be transferred between the load port and the process tool below the shelf. In alternative embodiments, the shelf may be mounted in the process tool so that a cassette stored on the shelf may be quickly transferred into a processing position within the process tool upon completion of processing on a prior wafer lot. In another alternative embodiment, the storage shelf may be located within load port minienvironment, directly behind the port door, on a side of the port door opposite the access port to the process tool. It is understood that the minienvironment may include shelves in other locations, as well as more than one storage shelf. In a still further alternative embodiment, grippers may be provided on a lower surface of the port plate for gripping and storing a cassette as processing on the first cassette takes place.
In operation, while a first cassette is located within the process tool, a second pod is seated on the load port, the cassette is separated from the pod and the cassette is stored on the storage shelf. When processing on the first cassette is completed, the second cassette is loaded into the processing tool. The first cassette is returned to the pod of the second cassette and removed from the load port. A cassette from a new pod is then seated on the load port, separated from its pod and stored on the storage shelf. This process continues until processing on each scheduled cassette is completed. By providing a buffer of cassettes within the SMIF interface and/or process tool, the processing tool is no longer dependent on timely delivery of pods to the interface to ensure that it does not sit idle.
It is known to be able to track a particular workpiece lot through a fab via SMART tag or similar technologies, in which information relating to a particular workpiece lot is transmitted to and stored in a tag affixed to the pod in which that lot is carried. According to the present invention, a workpiece lot is brought to a load port in a first pod and transported away in a second pod. In embodiments of the invention utilizing SMART tag or similar technologies, the information stored in the tag on the second pod may be rewritten before the second pod with the new workpiece lot leaves a load port so that the second pod accurately identifies the new workpiece lot stored therein.